The present invention relates to an image forming apparatus such as copier, multi-functional apparatus, laser printer, facsimile and so forth that performs image forming according to the electrophotography method, and to an image quality adjusting method.
In an image forming apparatus, density of a toner image that is deposited onto a revolving image bearing member may change caused by a change of environment such as temperature and/or humidity, and/or a change over time and so forth. Therefore, where necessary, density adjustment for a gradation correction is performed by carrying out a process control, that is, by forming a test pattern consisting of multiple patches sequentially in different levels of toner density on the image bearing member. More specifically, the process includes detecting the levels of density of the test patches formed on the image bearing member, and changing the image forming conditions such as developing bias and/or the like based on the detected values so that the levels of density agrees with an ideal gradation characteristic.
On the other hand, facing a revolving surface of the image bearing member, the image bearing member, together with an electrostatic charging portion to perform electrostatic charging, a laser exposure portion to form an electrostatic latent image, a developing portion to render the electrostatic latent image manifest by depositing thereto a toner and so forth, constitutes an image forming portion. If mechanical misalignment or a change such as eccentricity and/or the like occurs in a revolving shaft of the image bearing member, distance from the surface of the image bearing member changes in a sub-scanning direction which is rotational direction, and thus unevenness may occur in electrostatic charging characteristics, amount of laser light and toner deposition characteristics. Such mechanical misalignment or a change results in a change of toner deposition amount that is finally deposited onto the image bearing member, thereby lowering the reproducibility of images.
JP 2012-230312A describes an image forming apparatus in which a toner pattern is formed as a preprocessing on a circumferential surface of the image bearing member and, based on the result detected on period or the like of the toner pattern, a period with which a maximum amount of density change appears is determined and, based on the determination result, an arrangement of patches at the time of process control is decided to thereby offset the effect of the density change.
However, in the image forming apparatus described in JP 2012-230312A, it is necessary to decide the arrangement of the patches on the image bearing member depending on the period which is the determination result, and thus, at the time of an image quality adjustment, it is necessary to wait until the position decided on the image bearing member revolves to come to a position that is faced by the density sensor; therefore, there is a problem that doing so takes a time. Accordingly, the time needed to perform an entire image quality adjustment becomes longer. Moreover, in the case of color, because there are four colors, for each of which it is necessary to adjust a phase, the time needed to perform an overall positional adjustment cannot be ignored. Besides, because the image bearing member has to be revolved more to that extent, there is also a problem that its service life becomes shortened.